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8
SECOND REPORT
TO THE
MAIN DRAINAGE COMMITTEE OF THE LONDON COUNTY COUNCIL,
on
SEWER AIR INVESTIGATIONS.
By J. PARRY LAWS, F.I.C.
To the Main Drainage Committee.
Gentlemen,
In a former report submitted to your Committee an account was given of three series of
experiments on sewer air, together with their results.
The three series in question were experiments,
Firstly, On normal sewer air;
Secondly, On moderate splashing; and
Thirdly, On the value of various deodorants.
Their results, briefly summarised, were to the following effect—
Firstly—When the air within the sewer was compared with the air outside in immediate relation
with the sewer it was found that the bacteria present in the former were fewer in number, and that the
carbonic acid was from two to ten times as great as in fresh air. No sulphuretted hydrogen was found.
Secondly—Moderate splashing carried out so as to imitate the inflow of a lateral drain or house
sewer produced no variation in the sewer air even within such a short radius as four feet from the
disturbance.
Thirdly—When deodorants were added to the sewage, or distributed in the sewer air, no effect
was produced on the sewer air beyond the removal of, in most cases, the disagreeable smell. In some
instances, however, the deodorants when added to the sewage had a marked effect upon the sewage itself,
reducing very considerably the number of bacteria present.
As to the character of the micro-organisms, those found in the sewer air are related to those that
occur in fresh air, and consist chiefly of moulds and micrococci. On the contrary, those found in sewage
are chiefly bacilli, and not so related.
The present report deals with experiments directed to ascertain the following points—
1. Whether an increase in the velocity of the air current in a sewer beyond the limits
met with under ordinary conditions would produce a concomitant increase in the number of
micro-organisms.
2. Whether the conclusions arrived at from the experiments in large sewers hold good
for small sewers when the flow of sewage is intermittent, and the velocity of the air current
variable.
3. What influence, if any, stagnant and highly putrescent sewage has upon the number
of micro-organisms in sewer air.
4. Whether the various micro-organisms collected from sewer air during the progress of
all my experiments were identical with the micro-organisms isolated from fresh air in the same
vicinity and at the same time.
1.—Influence of velocity experiments.
Table I.—For the purposes of the first series of experiments, observations were made in the air
shaft erected at Pimlico pumping-station with the object of ventilating the low-level sewer. Through
the ventilating shaft, which feeds the boiler furnaces and which has a diameter of 2 feet, sewer air is
drawn at a speed varying from 5 to 11 feet per second according to the condition and number of fires in
use. Access was gained to the interior of the shaft by means of a small manhole situate about 60 feet
from the furnace end of the shaft. The following determinations were made in the sewer air and fresh
air as near as possible at the same time, viz., moisture, carbonic acid, number of micro-organisms,
temperature, and velocity of the air current. In these experiments, as well as those to be subsequently
described, nearly all the estimations were made in duplicate. In determining the number of microorganisms,
one sterile tube was placed transversely in the air shaft, the other horizontally with its open
end directed towards the air current.
On reference to the table (Table I.), it will be seen that the velocity of the air current varied
from a minimum of 5 feet to a maximum of 11 feet per second—the minimum of 5 feet in these experiments
coincides with the maximum velocity mentioned in my previous report. In experiments 1 and 2
it was impossible, owing to the bad weather, to make any comparison with fresh air; it will be seen,
however, that the number of micro-organisms is low in both instances ; the carbonic acid varied from
5.91 vols. to 28.25 vols. per 10,000. The results conclusively show that a considerable increase in the
velocity in the air current does not produce an increase in the number of micro-organisms found in the
sewer air.
2.— Experiments with 9-inch saver.
Table II.—Owing to the difficulty of gaining access to such a small sewer when in actual use,
an experimental sewer of the above dimensions and 80 feet long was laid in a suitable position in the
pump room at Pimlico station (diagram 1).